1. Field of the Invention
The invention relates generally to the field of subsurface rock formation productivity estimation. More specifically, the invention relates to methods for determining “skin” damage of a formation proximate a wellbore so that expected fluid production can be predicted.
2. Background Art
Wellbores are drilled through subsurface rock formations to extract useful substances such as oil and gas. A wellbore forms a hydraulic conduit from a permeable subsurface rock formation having oil and/or gas present therein to the Earth's surface. Oil and/or gas typically move to the surface through the wellbore by the force of gravity. Gravity manifests itself as a pressure drop between the fluid pressure in the pore spaces of the subsurface rock formation and the wellbore. The rate at which the oil and/or gas move into the wellbore and to the surface depends on the pressure drop between the formation and the wellbore, the viscosity of the oil and/or gas, and the permeability of the rock formation to the oil and/or gas.
It is known in the art that the permeability of a rock formation can be affected by the process of drilling a wellbore therethrough. Such effects can result from migration of small particles in the fluid (“drilling mud”) used to drill the wellbore, reaction of certain formation minerals (e.g., clay minerals such as kaolinite and chlorite) disposed in the pore spaces with the liquid phase of the drilling mud and/or mechanical and chemical alteration of the formation by the action of drilling the wellbore. A typical effect is to reduce the permeability of the rock formation proximate the wellbore. Such near-wellbore permeability reduction is referred to as “skin effect” or “skin damage” and may result in lower oil and/or gas flow rates than would be expected for the particular rock formation and/or higher pressure drop from the formation to the wellbore.
For certain formation evaluation procedures, for example, formation fluid testing using wellbore instruments conveyed into the wellbore, existence of skin damage may result in test failure or false indication that a particular formation is not likely to be productive of oil and/or gas. The existence of skin damage may be confirmed by more extensive testing of the formations, and remedial operations can be performed to overcome the production rate loss resulting from skin damage, however there exists a need to evaluate possible skin damage quickly and efficiently so as to reduce the number of formations improperly identified as non-productive, to reduce the number of formation tests that are failure prone and to identify subsurface formations that may benefit from remedial operations to correct skin damage.